The present invention relates to an impact drill for boring a hole in a concrete, mortar and tiles, and more particularly, to such impact drill providing a drilling mode in which a boring is performed by rotating a drill bit and a impact drilling mode in which boring is performed by rotating and impacting or vibrating the drill bit.
A conventional impact drill of this type is shown in FIGS. 15 through 18. A main frame 401 includes a gear cover 417, an inner cover 418, an outer cover 419, a housing 407, and a handle portion 406 connected thereto, those defining an outer configuration of the drill and housing therein various components at given positions. A spindle 402 extends through the gear cover 417, and a drill chuck 403 is attached to a front end of the spindle 402. The spindle 402 has an intermediate portion provided with a rotatable ratchet 404 rotatable together with the rotation of the spindle 402 and movable together with an axial displacement of the spindle 402. The rotatable ratchet 404 has one side 404a formed with a serration or alternating projections and recesses.
A fixed ratchet 405 is disposed in confrontation with the rotatable ratchet 404, and has a side 405a formed with a serration or alternating projections and recesses. The fixed ratchet 405 has a hollow cylindrical shape and is fixed at a position regardless of the rotation and axial displacement of the spindle 402.
Meanwhile, a motor 408 is disposed within the housing 407. The rotational driving force of the motor 408 is transmitted through a rotary shaft 409 to a gear 410. The gear 410 is force-fitted into a pinion 411, so the aforementioned rotational driving force is transferred to the pinion 411. The pinion 411 has two pinions 411a and 411b those having numbers of teeth different from each other and which are meshedly engaged with a low speed gear 412 and a high speed gear 413, respectively. When the pinion 411 rotates, the gears 412 and 413 rotate as well. These gears 412 and 413 are formed with concave portions.
A clutch disc 414 is disposed over and engages the spindle 402, and is slidable in an axial direction thereof. As shown in FIG. 15, when the clutch disc 414 is slidingly moved and pressed into the concave portion of the low speed gear 412, the rotation of the pinion 411 is transferred to the spindle 402 through the low speed gear 412 and the clutch disc 414. On the other hand, if the clutch disc 414 slides rightward from the position in FIG. 15, and when inserted into the concave portion of the high speed gear 413,the rotation of the pinion 411 is transferred to the spindle 402 through the high speed gear 413 and the clutch disc 414. Consequently, the spindle 402 can be given low-speed rotation or high-speed rotation based on the movement of the clutch disc 414.
A change lever 415 is provided for changing operation mode of the impact drill between a drilling mode and an impact drilling mode. A change shaft 416 is force-fitted into the change lever 415. By rotating the change lever 415 about its rotation axis, the change shaft 416 is rotated about its axis along with the change lever 415. As shown in FIGS. 16 through 18, the change shaft 416 is formed with a notch 416a. The impact drill operates in drilling mode when the notch 416a is in the position in FIG. 16, and operates in impact drilling mode when the notch 416a is in the position in FIG. 17.
Drilling mode will be described. If the bit (not shown) attached to the drill chuck 403 is brought into contact with a workpiece (not shown), and the handle 406 is pressed in the direction of the arrow in FIG. 15, and if the notch 416a in the change shaft 416 is in the position shown in FIG. 16, an internal end of the spindle 402 will abut against the outer peripheral surface of the change shaft 416 and will not be able to move rightward any more. As a result, the contoured serrated surface 404a of the rotation ratchet 404 and the contoured serrated surface 405a of the fixed ratchet 405 will not come into contact. Consequently, the rotational driving force of the motor 408 is transferred through the low speed gear 412 or the high speed gear 413 to the spindle 402, and only the rotational force is imparted to the bit.
In case of the impact drilling mode, the change lever 415 is rotated about its axis so as to displace the position of the notch 416a in the change shaft 416 to the position shown in FIG. 17. In this state, if the bit attached to the drill chuck 403 is brought into contact with the workpiece, and if the handle 406 is pressed in the direction of the arrow in FIG. 15, the inner end of the spindle 402 will enter the notch 416a as shown in FIG. 18. In other words, since the spindle 402 can be moved rightward slightly, the contoured surface 404a of the rotation ratchet 404 resultantly comes into contact with the contoured surface 405a of the fixed ratchet 405.
When drilling into the workpiece, if the spindle 402 is rotated in the state shown in FIG. 18, the rotatable ratchet 404 engages the fixed ratchet 405, so that vibration is generated by the pressure contact between the alternating projections and recesses of the serrated surfaces 404a, 405a of both of the ratchets 404 and 405, and this vibration is transmitted through the spindle 402 to the bit (not shown). In other words, rotational force and vibration are imparted to the bit, and drilling is performed by the combined rotational force and the vibration force.
However, when the vibration drill described above is operated in the impact drilling mode, the vibration is transferred not only to the bit, but also to the handle 406 by way of the fixed ratchet 405, the inner cover 418 and the housing 407. This leads to the problem that a large amount of vibration is passed to users of the impact drill, thus causing discomfort. In particular, if the impact drill is used continuously for long periods of time, caution must be exercised such that there are no adverse effects on the health of users.
Several proposals have been made for mechanisms to reduce the vibration passed to the users. For example, according to laid open Japanese utility model application publication No.S59-69808, as shown in FIG. 19, a spindle 520 is rotatably and axially movably supported to a housing through a bearing 511. A rotation cam 521 is fixed to the spindle 520, so that the rotation cam 521 is rotated together with the rotation of the spindle 520 and movable together with the spindle 520. A serrated contour is formed on a cam surface 521a of the rotation cam 521.
A clutch cam 522 is supported on a spindle 520 and is slidably movable in the axial direction of the spindle 520. The clutch cam 522 includes a hollow cylindrical section slidable with respect to the spindle 520, and a flange section 522b. A serrated contour is formed on a cam surface 522c of the flange section 522b. Further, a regulation slot 522a is formed at an outer peripheral surface at a position near a rear end portion 522d of the hollow cylindrical section. A plate 524 extending perpendicular to the spindle 520 is engaged with the regulation slot 522a. A spring 523 is interposed between the flange section 522b and the plate 524.
The spring 523 continuously urges the clutch cam 522 toward the rotation cam 521, and the cam surfaces 521a and 522c are pressed together when the spindle 520 is retracted into the housing. Then, when the force applied to the spindle 520 surpasses the biasing force of the spring 523, the spring 523 is compressed and the clutch cam 522 retracts (moves rightward in FIG. 19). However, the displacement of the clutch cam 522 is limited within a length of the slot 522a. When the clutch cam 522 moves forward from the retracted position by the biasing force of the spring 523, the clutch cam 522 strikes against the rotation cam 521, and the rotation cam 521 vibrates along with the spindle 520.
Since the vibration arising from the contact between the cam surfaces 521a and 522c is alleviated by the spring 523 before being transmitted to a handle (not shown), the mechanism shown in FIG. 19 is advantageous in reducing the transmission of vibration to the user in comparison with the mechanism shown in FIG. 15 where the ratchet 405 is placed in a fixed position.